prepayment vs retainer

prepayment

noun
  • A payment in advance. 

  • An asset recognized in respect an expense incurred in a period for which the benefit will be received in a future period. 

retainer

noun
  • A fee one pays to reserve the other's time for services. 

  • A device that holds teeth in position after orthodontic treatment. 

  • Any thing or person that retains. 

  • A dependent or follower of someone of rank. 

  • A retaining valve. 

  • A paid servant, especially one who has been employed for many years. 

How often have the words prepayment and retainer occurred in a corpus of books? (source: Google Ngram Viewer )